The invention concerns a process for x-ray exposing a radiation sensitive layer.
During the manufacture of integrated semiconductor circuits, the semiconductor wafer areas to be processed in each process step are defined by photoresist layers which are exposed in the form of the patterns desired in each case and which are subsequently developed. Increasing miniaturization has caused the maximum 2 .mu.m resolution of the previously employed exposure processes, using radiation wavelengths ranging from 0.3 .mu.m to 0.7 .mu.m, to be reached and exceeded in some cases. Recently there has been a tendency to use processes in which exposure in effected by electron beams or x-rays.
However, in the case of electron beam or x-ray exposure the resolution is very high. Theoretically lines with widths of 0.1 .mu.m and less can be transferred, but both electron beams and x-rays have a number of disadvantages which limit their use for the industrial production of integrated circuits. The essential disadvantages inherent in the use of electron beams are due to the fact that the electrons are scattered in the radiation sensitive layer. Therefore, particularly in the case of narrow structures, a considerable widening of the exposed areas occurs. This phenomenon is particularly detrimental at very small distances between the individual exposed areas, since the undesired and inaccurately defined widening may lead to contacts between adjoining structures, thus rendering the finished integrated circuits unserviceable. Only elaborately computed pattern or exposure dose variations ensure that the desired dimensions of the patterns are actually obtained after development. The fact that the value 1 cannot be exceeded for the ratio of the photoresist layer thickness to the smallest line width is also attributable to the scattering of the electrons. Because, for the etching or implantation steps following exposure and development, photoresist layers with a thickness of at least 1 .mu.m are indispensable, the use of electron beams for submicron lithography is practically precluded.
The aforementioned disadvantages are not encountered when x-rays are used to expose photoresist layers, because during x-ray exposure the line width to the photoresist layer thickness ratio may be less than 1:10. However, x-rays cannot be bundled into a bunch of parallel rays, and the x-ray sources available generate ray cones whose solid angles are about 60.degree.. A consequence of this is that at distances from the x-ray source at which an approximately parallel radiation is present, the intensities are so low that exposure times of many hours and even days are required. Exposure times of such length are considerably extended by the fact that there are no mask substrates which even at thicknesses of 3 .mu.m and less pass more than 50% of the x-radiation. Because of this it is not possible during the manufacture of integrated semiconductor circuits to x-ray expose only small areas, for example, chip areas. For exposure to be relatively economical, it is necessary to utilize the full ray cone of the x-ray source, i.e., to simultaneously expose several hundred semiconductor wafers, i.e., 20,000 to 100,000 chips. The exposure of only small areas of the semiconductor wafer, e.g., areas having the size of one or several chips, which is desirable per se, is not economically acceptable because of the plurality of highly accurate alignments required and because of the poor utilization of the x-ray cone. Because of the inevitable warping of the wafer, the simultaneous x-ray exposure of a whole wafer necessitates distances between mask and wafer of at least 30 to 50 .mu.m. The number of faulty exposures is so high in practice that the simultaneous exposure of several hundred semiconductor wafers is not possible during the industrial production of integrated semiconductor circuits with submicron structures. Although the lateral displacement of a mask image transferred by obliquely incident beams, which in the case of highly divergent beams may differ considerably from one location to another, can be compensated by suitable measures during mask manufacture, it must not be overlooked that reversible or irreversible local length changes of the masks and/or the semiconductor wafers or local alignment errors caused by local distance changes and amounting to several .mu.m are still encountered. This shows that the previously known methods of x-ray exposure cannot be used for the large-scale industrial production of integrated semiconductor circuits with line elements in the submicron range, although during the development of x-ray exposed photoresist layers channels with perpendicular smooth walls can be generated, whose width is less than a tenth of the thickness of the photoresist layer. Alignment methods for x-ray lithography meeting the accuracy requirements of submicron lithography have not become known so far.
It is the object of the invention to provide a process for the exposure of light-sensitive layers, utilizing all the advantages of electron beam and x-ray exposure, while eliminating the disadvantages.